Measurements of the visual field are considered very important in ophthalmology in clinical practice and are utilized for following diseases and treatments in glaucoma, diabetes, other vascular diseases, uveitis, retinal detachment, neurological disorders like brain tumors, etc. Visual field examinations test differential light sensitivities at various locations within the visual field of the eye in a subject being tested. The visual field is defined as that portion of space that is visible to the fixated eye.
Perimetric and camperimetric tests are determinations of the character and extent of the indirect field of vision. The campimetric test, however, is confined chiefly to the central and para-central field. Campimetric studies provide a detailed examination of the part of the total visual field that extends within the 30 to 40 degree range of the point of fixation, in the center of the visual field. Studies of this portion of the visual field are especially important in all pathological conditions that affect the retino-cerebral system, and deficiencies of function manifested within this area possess great significance with regard to visual and ocular behavior. This area contains the macular area and the physiological blind spot,. Pathological conditions in the retina result in areas of non-vision or reduced sensitivity of vision, or total loss of vision. These areas of non-vision surrounded by areas of vision can take various shapes and sizes and are called scotomas.
There is no apparatus routinely used that is designed for the study of the field of vision that is capable of rendering an accurate depiction of the visual field when a central scotoma exists, i.e. a pathological area of non-vision that includes the exact center of the field. The primary advantage of the current invention is its ability to accurately map the size and shape of a central scotoma during a central visual field examination by maintaining fixation of the eyes. All known devices that measure visual field are completely dependent upon the patient's capacity to hold the eye steadily fixed upon a central point. If the pathological area of non-vision, or scotoma, covers the center of the field, the patient is unable to see the central fixation point and therefore cannot fixate on it and the device produces incorrect results. These devices, generically called visual field testers or automatic perimeters, are not adequate for testing any portion of the field of vision when a central scotoma exists in the exact center of the field.
Current commercial visual field testing devices are typically of the perimetric type, intended to map the entire visual field including the central field, though recently companies are producing models that measure only the central field but use the same principles as the full-field test. Generally speaking, current commercial visual field testing devices use the inside surface of a hemisphere, which provides for visual field testing with a movable flashing light to map a patient's visual field. In these instruments, the patient is situated in front of the instrument, and one eye is occluded. Each time a light flashes for the eye being tested, the patient indicates that the light is seen by pressing a button. A fixation light is provided in the center of the instrument to keep the patient's visual axis aligned with the instrument. The results are reported in the form of a two-dimensional ‘map’ of the patient's visual field showing clustered groups of missed light flashes as suspected scotomas or areas of disease. This device maps peripheral scotomas and has generally proven to be accurate, as it employs equipment that objectively monitors the point of fixation of the patient, and keeps a log of when the patient loses fixation on the central fixation target.
However, commercially available visual field testing instruments fail to produce accurate results when the patient has a central scotoma. The obvious problem is that when clinicians want to map such a field, they will ask the patient to stare at the central fixation target, which is impossible for these patients since that is exactly where they are blind. Typically in the case where the scotoma is small and covers the central fixation point, the patient will shift his gaze so he can see the fixation light, in which case he is no longer looking at the fixation light with the center of his vision, and continue with the test. This produces incorrect mapping of the entire field, and a central deficiency is reported as being located off-center when it is not, or can be missed altogether. For patients with a large central scotoma, it is likely that it will be impossible for them to ever see this fixation light and shifting their gaze will not help. A patient with large central scotoma (i.e. 40 degrees in extent) is not a candidate for a commercial visual field tester due to the fact that the patient cannot fixate. Lack of fixation would give each data point on the resulting chart its own random error of offset, depending on the direction of the eye when that point was tested, which would make the results unusable. Therefore, the ability to keep the patient's eye centered and motionless during a test is critical for any visual field-measuring device.
Some manufacturers of commercial instruments have recognized this problem of trying to fixate when an area of non-vision exists at the point of fixation, and have attempted to solve it by providing multiple fixation points arranged in some pattern around but not in the center of vision. The patient is instructed to look in the center of say four fixation points which form a square surrounding the central point of vision. The results of multiple fixation points tends to fail almost as consistently as with one fixation point, since often the scotoma will be large enough that these points cannot be seen either, or the patient may only see three of the four points due to a small scotoma, or it may just be too confusing or tiresome for the patient to bother.
The problems associated with monocular visual field testing was approached in the 1920's by Kirk, who reasoned that if the eye under test had a central scotoma and the other eye did not (which is often the case), the other eye could be used to aid in fixation using binocular vision.
Kirk suggested the use of a central target within the testing area for the eye under test and an identical target to be viewed by the eye not under test. The eye not under test would view the target by way of a mirror placed before it, so that the object was out of the way of the testing arena. He also suggested the use of stereoscopic targets, or one that would appear to have depth when the two images were fused. The eye not under test is able to fixate properly on its own fixation target, if it has no central scotoma, so it holds the eye under test fixated because of the natural tendency to maintain fusion. In this manner, the visual field could be obtained even though the eye under observation is so impaired that it cannot directly see the fixation target.
In the late 1920's and 1930's Lloyd developed a stereocampimeter, which used the principles in Kirk's U.S. Pat. No. 1,735,005, namely binocular stereoscopic fixation. The instrument was manufactured by American Optical Company and was intended for use at reading distance. The system contained a stereoscope (two images taken from different perspectives) for fixation. The testing arena was a flat area extending 30-40 degrees of the field of vision. The test used a piece of black paper that had non-linear grid lines printed on it representing the curvature of the retina projected on a two-dimensional surface. The eye under test was to look at a central feature on the paper, and the other eye was redirected with a mirror to another surface that had the same central feature and grid lines, yet was displaced slightly relative to the testing arena. Once the two images were aligned with each other it would appear to the patient that he were looking at one fixation point and one set of grid lines which had depth. The examiner would then insert a test object into the field of vision. This was accomplished by a using a white (or colored) sphere (or disc) attached to a thin wire wand. The size of the sphere determined the angular subtense of vision, and the wand was painted black and meant to be invisible. The examiner would insert the test wand throughout the field of vision and ask the patient whether it was visible. If the patient indicated it was not visible, the examiner would mark with a colored pencil on the black paper the points that the patient did not see. The test was painstakingly slow and distracting to the patient, and after complete, the examiner would connect the dots that were not seen obtaining a closed curve representing a scotoma.
Though the technique of stereocampimetry was novel, the device suffered from shortcomings which led to its extinction. The device required a skilled operator to perform the test, and was void of any technology. The patient who was required to concentrate and fix his gaze on the central fixation target while maintaining stereo fusion of the two images, was typically distracted by the examiner's hand coming in and out of the field to mark the points and place the targets. The patient had to be asked if a point was seen and had to respond with a yes or no answer. Since the black paper was illuminated with an incandescent light bulb, the wand was sometimes visible and also caused distraction. Also a significant problem was the time required to take the test, which can fatigue a patient, who is typically elderly with severe vision problems. The resolution of the test is dependent on the number of points the examiner samples would typically be low, and the examiner can never be sure the patient has kept his gaze fixed during the test which would invalidate the data. The other inherent problem with the device deals with the alignment of the stereo targets. If the patient is not fusing the two images, the alignment between the two images needs to be adjusted, which is difficult for the type of patient described above. Also, there were no means to ensure the patient was fixating therefore the test was unreliable.